A high-pressure discharge lamp is used as a light source in a projector-type image display apparatus such as a liquid crystal projector.
The high-pressure discharge lamp has a pair of opposing electrodes disposed inside an arc tube enclosing, for example, a halogen material, a noble gas, and mercury. As a lighting method, a predetermined high voltage is applied to the high-pressure discharge lamp to cause dielectric breakdown between the electrodes, and subsequently an alternating current of a predetermined frequency is caused to flow.
To lengthen the life of the lamp, a technology is known for controlling the shape of the electrodes as appropriate by switching the frequency of the alternating current while the lamp is lit (for example, see patent citation 1).    Patent Citation 1: Patent 2003-338394